The invention generally relates to biomedical sensors and relates in particular to biomedical sensors for detecting localized electrical signals within a subject.
Conventional disc biomedical sensors have generally changed little since Hans Berger first recorded the human electroencephalogram (EEG) in 1924. One drawback of conventional EEG methods that are recorded with disc electrodes, is that the procedure lacks high spatial resolution. This is primarily due to the blurring affects of the different conductivities of the volume conductor such as the cerebrospinal fluid, skull, and the scalp. Conventional EEG signals recorded with disc electrodes also have reference electrode problems as idealized references are not available with EEG. Placing the reference at different locations changes the characteristics of the EEG signals.
To increase the spatial frequency and selectivity the surface Laplacian has been utilized. Concentric ring electrodes automatically estimate the surface Laplacian significantly better than by processing conventional EEG signals (See “Development of Tri-Polar Concentric Ring Electrode for Acquiring Accurate Laplacian Body Surface Potentials”, by W. Besio, R. Aakula, K. Koka and W. Dai, Annals of Biomedical Engineering, Vol. 34, No. 3, March 2006) and significantly improves the signal-to-noise level in EEG applications, (see “Tri-Polar Concentric Ring Electrode Development for Laplacian Electroencephalography, by W. Besio, R. Aakula, K. Koka and W. Dai, IEEE Transactions on Biomedical Engineering, Vol. 53, No. 5, May 2006), as well as spatial selectivity, and mutual information (see “Improvement of Spatial Selectivity and Decrease of Mutual Information of Tri-Polar Concentric Ring Electrodes”, by K. Koka and W. Besio, Journal of Neuroscience Methods, Vol. 165, pp. 216-222, Jun. 9, 2007). The reference problem is alleviated as well since bipolar differences are taken at closely spaced electrode elements. The presence of hair however, remains a concern limiting the surface where the electrodes can make contact with the scalp without shaving.
Typically, an electrode gel (e.g., an electrolyte) has been used to bridge between electrodes and a cleaned surface of a subject (e.g., the scalp). The thickness of the gel varies with different applications, causing varying electrical properties with different applications. The gel also dries over time further changing the properties of the bridge distorting the EEG. Such gels must also be packaged in sealed packages to avoid becoming dried out prior to being used and may not be reused once opened. Gels may also irritate the scalp and/or the desired recording may be from a sensitive area of the subject, such as the eye, where the use of gel should be avoided. Further, if the spacing of electrodes is too small then the gel, which is typically an electrolyte, may directly connect the electrodes, shorting the sensor. Also, the application and removal of gels is time consuming.
Further, the spacing required between electrodes may be so small that smearing of the electrolyte (and thus short circuiting of the bioelectric signal) may occur. Additionally, and perhaps most importantly, the application and removal of electrolyte gels is an unpleasant process for the subject, and time consuming for the clinician or care giver. There are also toxicological concerns with electrolyte gels where dermatological responses are common.
To avoid the problems of electrolytes, dry electrodes (not using a gel) have been introduced. With dry electrodes, however, movement artifacts are more prevalent due to the absence of a thick electrolyte layer (as is present in gels, which provides a shock absorber function). The introduction of active electrodes (where buffering/amplification takes place at the electrode site) provides much less emphasis on the skin-electrode impedance. An added concern with dry electrodes is that the large RC constant, which exists at the input of the unity gain amplifiers typically used for this application, prolongs the effect of large artifacts.
There is a need therefore, for an improved biomedical sensor that may be used without the current drawbacks of using a gel yet may also provide consistent and reliable signals.